As discussed in Note 1, we primarily derive our revenues from the sales of various chemical products. The following table presents our net sales disaggregated by our principal markets, which disaggregation is consistent with other financial information utilized or provided outside of our consolidated financial statements:

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
	(Dollars In Thousands)
Net sales:
Agricultural products	$	35,494	$	35,998	$	154,790	$	146,291
Industrial acids and other chemical products		30,552		34,788		105,579		105,700
Mining products		9,449		8,995		30,805		31,439
Total net sales	$	75,495	$	79,781	$	291,174	$	283,430

For most of our contracts within the scope of Accounting Standards Codification, Revenue from Contracts with Customers (Topic 606) (“ASC 606”), the transaction price from the inception of a contract is constrained to a short period of time (generally one month) as these contracts contain terms with variable consideration related to both price and quantity. These contract prices are often based on commodity indexes (such as NYMEX natural gas index) published monthly and the contract quantities are typically based on estimated ranges. The quantities become fixed and determinable over a period of time as each sale order is received from the customer.

The nature of our contracts also gives rise to other types of variable consideration, including volume discounts and rebates, make-whole provisions, other pricing concessions, and short-fall charges. We estimate these amounts based on the expected amount to be provided to customers, which result in a transaction price adjustment reducing revenue (net sales) with the offset increasing contract or refund liabilities. These estimates are based on historical experience, anticipated performance and our best judgment at the time. We reassess these estimates on a quarterly basis.

The aforementioned constraints over transaction prices in conjunction with the variable consideration included in our material contracts prevent a practical assignment of a specific dollar amount to performance obligations at the beginning and end of the period. Therefore, we have applied the variable consideration allocation exception.

Future revenues to be earned from the satisfaction of performance obligations will be recognized when control transfers as goods are loaded and weighed or services are performed over the remaining duration of our contracts. Although most of our contracts have an original expected duration of one year or less, for our contracts with a duration greater than one year, the average remaining expected duration was approximately 14 months at September 30, 2019.

Our contract assets consist of receivables from contracts with customers. Our accounts receivable primarily relate to these contract assets and are presented in our consolidated balance sheets. Customer payments are generally due thirty to sixty days after the invoice date.

Our contract liabilities primarily relate to deferred revenue and customer deposits associated with cash payments received in advance from customers for volume shortfall charges and product shipments. We had approximately $3.8 million and $7.0 million of contract liabilities as of September 30, 2019 and December 31, 2018, respectively. For the three and nine months ended September 30, 2019, revenues of $1.2 million and $3.2 million, respectively, were recognized and included in the balance at the beginning of the respective period. For the three and nine months ended September 30, 2018, revenues of $0.8 million and $4.1 million were recognized and included in the balance at the beginning of each period.

Revenue recognized in the current period from performance obligations related to prior periods (for example, due to changes in transaction price) was not material.